1. Field of the Invention
The present invention relates to a disc cartridge comprising a recording disc rotatably enclosed in the cartridge, and more particularly to a disc cartridge mounted in a recording and play back device for recording and reproducing information.
2. Description of the Related Art
There is provided an optical magnetic disc into which an information signal can be repeatedly erased and rewritten using the thermal magnetic effect. In a production method of the optical magnetic disc, a pair of resin layers, wherein guide grooves for tracking and pits for addressing track addresses are formed, are transferred onto the surfaces of a pair of transparent substrates made of material such as glass etc., and a pair of vertical magnetic recording media, such as a sort of amorphous film, are respectively formed on the surfaces of the resin layers. An information signal can be erased and written into the vertical magnetic recording medium using the thermal magnetic effect.
The writing and erasing process into the optical magnetic disc will be described below. As shown in FIG. 1(a), a current flows through a magnetic coil 50 and a vertical magnetic recording medium is magnetized at a magnetic field intensity H in a predetermined magnetization direction. Next, as shown in FIG. 1(b), a laser light is radiated onto a recording portion of the vertical magnetic recording medium by a laser head 52 so as to heat the recording portion up to around Curie temperature, thereby the magnetization direction M is reversed by the thermal magnetic effect. The plane of the direct line polarization of the laser light is rotated by the Kerr effect when the laser light is reflected on the magnetic pole, wherein the rotation direction of the plane of the polarization depends on the magnetization direction M.
As shown in FIG. 2, a laser light emitted by a laser 56 is split by beam spliters 57 and 58 and received by an optical signal detecting system 59, on the other hand, the laser light with a rotated plane of the polarization passes through an analyser 54 into an optical detector 55, the optical detector 55 detecting the strength of the laser light directly proportional to the rotation angle of the plane of the polarization.
Referring back to FIG. 1(c), if the magnetization direction M of the vertical magnetic recording medium is rotated by a pattern corresponding to a predetermined information signal, each of the rotated or non-rotated portions of the magnetization direction in the vertical magnetic recording medium can be used as a bit for recording an information signal. On the contrary, the portion of the vertical magnetic recording medium where the magnetization direction is rotated in the opposite direction -M to a predetermined reference magnetization direction M, is magnetized in the opposite magnetization direction to the magnetized direction -M, heating the portion up to the Curie temperature, thereby the magnetization direction of the portion of the vertical magnetic recording medium become the reference magnetization direction M, resulting in the fact that the information signal of the portion of the vertical magnetic recording medium can be erased.
Therefore, as described above, an electromagnetic coil 50 for magnetizing the vertical magnetic recording medium at a predetermined magnetization intensity H and a laser head 52 for heating the vertical magnetic recording medium 51 up to the Curie temperature are required in the recording and play back device for the aforementioned optical magnetic disc. The electromagnetic coil 50 and the laser head 52 can be arranged side by side at the one side of the optical magnetic disc, however, from the view point of the ease of the production of recording and play back device, the laser head 52 is preferably arranged facing the bottom surface of the optical magnetic disc 60 and the electromagnetic coil 50 is preferably arranged facing the top surface of the optical magnetic disc 60, as shown in FIG. 2.
FIG. 3 shows a conventional disc cartridge rotatably comprising the aforementioned optical magnetic disc which was laid open in the Japanese Provisional Publication No. 150963/1983. In FIG. 3, there is provided a cartridge case 61 made of a synthetic resin, and an optical magnetic disc 60 rotatably arranged in the cartridge case 61, and a shutter 63 provided for opening and shutting a spindle insertion hole 66 and a head insertion hole 67, wherein the spindle insertion hole 66 and the head insertion hole 67 are formed on the surface of the cartridge case 61 as described below in detail.
As shown in FIGS. 3 and 4, the cartridge case 61 is a combination of a top half 64 and a bottom half 65 in the shape of a shallow square shape dish by facing the top and bottom halves 64 and 65 together, the aforementioned optical magnetic disc 60 and the other component member are arranged in a spacing formed in the inside of the cartridge case 61. The circular spindle insertion hole 66 is formed around the center portion of the top and the bottom halves 64 and 65 corresponding to the rotation center of the optical magnetic disc 60, and the head insertion hole 67 is formed at the portion extending in a radial direction from the outside of the spindle insertion hole 66 to the side rim portion of the cartridge case 61. A connecting portion 69 is formed between the side rim portion 68 of the cartridge case 61 at the insertion side into the recording and playing back device, corresponding the front rim portions of the top and the bottom halves 64 and 65, and the end portion 67a of the head insertion hole 67, and the shutter 63 is slidably arranged along the recess groove 70, formed in the shape of a direct line at the connection portion 69. The explanation of the details of the structure of the shutter 63 is omitted because the structure of the shutter 63 is not directly associated with the subject matter of the present invention.
In the aforementioned conventional disc cartridge 61, the shutter 63 is opened upon the insertion of the disc cartridge 61 into the recording and playing back device, and a spindle mounted in the recording and play back device is inserted into the spindle insertion hole 66, resulting in that the optical magnetic disc 60 is rotatably supported by the spindle. Next, as shown in FIG. 4, the electromagnetic coil 50 and the laser head 52 are inserted from the outside of the head insertion hole 67 formed on the surface of the top and bottom halves 64 and 65 into the portion facing the top and the bottom surfaces of the aforementioned optical magnetic disc 60, thereby the recording and playing back operations can be performed.
In the aforementioned conventional disc cartridge, the method for arranging the electromagnetic coil 50 and the laser head 52 at a predetermined position facing the optical magnetic disc 60 has been adopted as follows, since the connecting portion 69 is formed at the side rim portion 68 of the top and bottom halves 64 and 65. As shown in FIG. 5, the electromagnetic coil 50 and the laser head 52 must be arranged so as to move up and down for facing the optical magnetic disc 60 in the recording and playing back device, thus, the cartridge case 61 is inserted between the electromagnetic coil 50 and the laser head 52 on the condition that the electromagnetic coil 50 and the laser head 52 are arranged at a predetermined spacing. After that, as shown in FIG. 4, it is necessary to insert the electromagnetic coil 50 and the laser head 52 into the inside of the cartridge case 61 so that the electromagnetic coil 50 and the laser head 52 face the optical magnetic disc 60. Therefore, there is a problem that the recording and playing back device become relatively large since the carrying mechanism for supporting and moving the electromagnetic coil 50 and the laser head 52 is complicated. A further problem is in that the electromagnetic coil 50 and the laser head 52 can not be positioned precisely with respect to the optical magnetic disc 60.